1. Field of the Invention
The invention relates to emulsions of the water-in-oil (W/O) or oil-in-water (O/W) type and the preparation thereof.
2. Description of the Related Art
Emulsions, either as water-in-oil (W/O) or oil-in-water (O/W) dispersions, are widely used as an application form for coating materials, such as, for example, water-based paints and finishes, as adhesives and sealants, such as, for example, aqueous epoxy or polyurethane systems, as cosmetic formulations, as cleansing agents and disinfectants, in the food industry, for the surface modification of solid or liquid substrates or as reaction media in emulsion polymerization.
In general, the dispersing and stabilization of the disperse phase are effected with the aid of emulsifiers. Cationic, anionic, ampholytic and nonionic emulsifiers are used. Common to the emulsifiers is that they are surface-active substances. That is to say, they preferably accumulate at interfaces, such as, for example, liquid-liquid, liquid-solid or liquid-gas interfaces, and thus reduce the interfacial/surface energy. On application of the emulsion, however, the emulsifiers can also cover the surface of the substrate to be treated and thus greatly change the wetting properties of the surface. This can adversely affect, for example, the adhesion properties of a coating material or of an adhesive joint or seal. Furthermore, the recoatability may be adversely affected. In addition, emulsifiers based on organic molecules are potential hazardous substances when used in pharmaceutical or cosmetic formulations or in foods.
In 1907, Pickering described for the first time the preparation of emulsions which were stabilized only by addition of various solids, such as basic copper sulfates, basic iron sulfates or other metal salts. This type of emulsion is also referred to as a “Pickering emulsion”. Basic investigations showed that a characteristic of Pickering emulsions is that solid particles are arranged at the interface between the two liquid phases and form a barrier there to the coalescence of the disperse phase.
Frequently, however, such solid-stabilized emulsions as described, for example, in EP 987008 have a high viscosity and/or a great tendency for separation, i.e. for creaming or sedimentation of the disperse phase.